Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
DHW Demand
Attach temperature probes as near as possible to plate heat exchanger flow and returns (indicated by red arrows)
User Control settings for DHW to MAX
Gas On
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Measure the temperature differential is there between 10°C and 20°C approximately?
Note:
The fan and burner have been removed for clarity but the test is carried out with these in place.